Culture, Bacterial Microbiology & Sensitivity

Using a specially packaged sterile cotton culture swab, a sample is carefully taken of a body fluid or a surface. The swab is introduced back into a sterile “growth media”, which is a gel or liquid environment that the bacteria will thrive in until the sample is tested at the laboratory.

Once the sample arrives at the laboratory, the swab is removed from the media and spread out on a bacterial growth medium (agar) in a petry dish. The growth patterns and characteristics of bacteria when given different nutrients are the basis of microbiology. A “sensitivity” is determined when discs infused with different relevant antibiotics are placed on the bacterial colony.

The effectiveness of the antibiotic against the bacteria is measured by the size of the zone in which bacteria die or do not grow.

Benefits

Bacteriologic techniques are our means for determining the specific bacteria causing infection. In horses, culture is commonly used to differentiate bacterial uterine infections.

Limitations

Bacterial growth may be slow. It can take days in the laboratory to obtain a final culture. Some organisms are fastidious and may not grow at all.

Contaminant organisms may cloud the picture. There may be more dominant species of bacteria that overgrow the sample, preventing the growth and identification of the most important bug.

Time delay due to offsite evaluation. False positives come from contaminants that grow in the laboratory, confusing the picture. False negatives comes from bacteria that are a problem for the horse but do not grow. Your vet must decide what they think is significant and requires treatment.

QUESTIONS TO ASK MY VET

What is the likelihood in this case that the culture result is significant?

Is the result of this test going to change the way you treat my animal?

Author: Doug Thal DVM Dipl. ABVP

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